The long-term objective of this proposal is to understand the molecular mechanism of oncogenic transformation by SV40 by analyzing the structure and function of the viral DNA molecules. This problem will be approached in two ways: 1) The arrangement of SV40 genes and control elements in the SV40 genome will be analyzed by application of electron-microscopic heteroduplex mapping techniques to certain Adenovirus-SV40 hybrids and SV40 deletion mutations; 2) The fate of SV40 DNA molecules during abortive (transforming) infection will be examined in order to define the intracellular events which lead to integration of the viral genome into the host genome and the cellular factors which affect the efficiency of integration.